Early in the development of prestressed concrete, the goal of prestressing was the complete elimination of concrete tensile stress at service load. This kind of design, in which the service load tensile stress limit fls = 0. is often referred to as full prestressing.
While full prestressing offers many advantages over nonprestressed construction, some problems can arise. Heavily prestressed beams, particularly those for which full live load is seldom in place, may have excessively large upward deflection, or camber, which will increase with time because of concrete creep under the eccentric prestress force. Fully prestressed beams may also have a tendency for severe longitudinal shortening, causing large restraint forces unless special provision is made to permit free movement at one end of each span. If shortening is permitted to occur freely, prestress losses due to elastic and creep deformation may be large. Furthermore, if heavily prestressed beams are overloaded to failure, they may fail in a sudden and brittle mode, with little warning before collapse.
Today there is general recognition of the advantages of partial prestressing. in which flexural tensile stress and some limited cracking is permitted under full service load. That full load may be infrequently applied. Typically, many beams carry only dead load much of the time, or dead load plus only part of the service live load. Under these conditions, a partially prestressed beam would normally not be subject to flexural tension, and cracks that form occasionally, when the full live load is in place, would close completely when that live load is removed. Controlled cracks prove no more objectionable in prestressed concrete structures than in reinforced concrete structures. With partial prestressing, excessive camber and troublesome axial shortening are avoided. Should overloading occur, there will be ample warning of distress, with extensive cracking and large deflections (Refs. 19.10 to 19.13).
Although the amount of prestressing steel may be reduced in partially prestressed beams compared with fully prestressed beams, a proper safety margin must still be maintained, and to achieve the necessary flexural strength, partially prestressed beams may require additional tensile reinforcement. In fact, partially prestressed beams are often defined as beams in which (a) flexural cracking is permitted at full service load and (b) the main flexural tension reinforcement includes both prestressed and nonprestressed steel. Analysis indicates, and tests confirm, that such nonprestressed steel is fully stressed to/v at flexural failure.
The ACI Code does not specifically mention partial prestressing but does include the concept explicitly in the classification of flexural members. Class T flexural members require service level stress checks and have maximum allowable tensile stresses above the modulus of rupture. Class C flexural members do not require stress checks at service load but do require crack control checks (Section 19.18). The designations
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of Class T and Class C flexural members brings the ACI Code into closer agreement with European practice (Refs. 19.13 to 19.15).
The three classes of prestressed flexural members. U, T, and C, provide the designer with considerable flexibility in achieving economical designs. To attain the required strength, supplemental reinforcement in the form of nonprestressed ordinary steel or unstressed prestressing strand may be required. Reinforcing bars are less expensive than high-strength prestressing steel. Strand, however, at twice the cost of ordinary reinforcement, provides 3 times the strength. Labor costs for bar placement are generally similar to those for placing unstressed strand on site. Similarly, the addition of a small number of strands in a plant prestressing bed is often more economical than adding reinforcing bars. The designer may select the service level performance strategy best suited for the project. A criterion that includes no tensile stress under dead load and a tensile stress less than the modulus of rupture at the service live load is possible with Class U and T flexural members, while Class C members use prestressing primarily for deflection control.
The choice of a suitable degree of prestress is governed by a number of factors. These include the nature of the loading (for example, highway or railroad bridges, and storage warehouses), the ratio of live to dead load, the frequency of occurrence of the full service load, and the presence of a corrosive environment.
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